Davidson's locomotive: how did he do it?

In about 1840, when electric motors were little more than scientific toys and the electric battery only a laboratory device, an obscure Scotsman built an electric locomotive weighing some five tons. Powered only by the most primitive of batteries it carried two people for a mile and a half at speeds upwards of four miles an hour. In the cold light of today's knowledge, it appears an impossibility but the reports are beyond question. This paper describes experiments with a small-scale model which suggest some answers.